Re-Evaluating How Vegetables Are Stored

MSU AgBioResearch food scientists Elliot Ryser, Eva Almenar, Janice Harte and Randy Beaudry, together with colleagues from Rutgers University and Ohio State University, are studying the impact and increasing the efficacy of sanitizers and gases used in packaging fresh-cut produce through a $2 million grant from the U.S. Department of Agriculture (USDA) National Institute of Food and Agriculture.

Chemical compounds are used to treat produce before packaging to neutralize any harmful pathogens that may be present, such as Salmonella or E. coli. The use of these sanitizers is required by federal regulation before the produce can be sold. Most produce packages are also filled with gases, such as carbon dioxide, to inhibit the growth of microbes. The MSU team was tasked with finding the right balance between the compounds and gases to ensure maximum food safety without sacrificing freshness.

A series of pathogen outbreaks and product recalls — most notably a 2006 E. coli outbreak in fresh spinach that resulted in over 270 hospitalizations and three deaths — were the impetus for this project.

“This is part of a much larger USDA project to look at safety in fresh fruits and vegetables,” said Ryser, professor in the MSU Department of Food Science and Human Nutrition. “After being treated with sanitizers, many products are packaged under various atmospheres and exposed to fluctuating temperature conditions, which can increase the chances that they become contaminated or lose their freshness. Our findings will be built into USDA’s risk assessment program to improve food safety.”

Washing and sanitizing fresh produce

Combating pathogens begins as soon as the produce is harvested. The produce is washed before packaging with water that has been mixed with sanitizing agents such as chlorine and peracetic acid to remove pathogens. These compounds remove 90 percent to 99 percent of the pathogens from the produce itself. Though that eliminates the vast majority of pathogens, a colony of 1,000 Salmonella cells on a single tomato is capable of contaminating an entire batch. Ryser’s lab studied this spread of pathogen from inoculated to non-inoculated produce in tomatoes, onions, celery and cantaloupe.

Ryser and his team found that controlling the temperature at which produce is stored is critical to controlling pathogen spread. The produce can undergo significant temperature fluctuations during transportation from the field to the store. Often vegetables are briefly kept in warmer areas such as loading docks or unrefrigerated rooms for cleaning, during which time pathogens — which may have been present in non-harmful quantities — can rapidly proliferate.

“If the temperature increases, you have the potential for pathogen growth,” Ryser said. “Retailers recognize there are issues here, and our research is going to help them resolve those issues.”

Packaging to inhibit microbial growth

Controlling the climate in which produce is stored is only one part of the solution. Almenar, associate professor in the MSU School of Packaging, worked with her team to evaluate the in-package gases used to inhibit microbial growth in produce.

“To minimize microbial growth of both pathogens and molds, we use in-package gases with lower oxygen and higher carbon dioxide,” Almenar said. “Such mixtures can reduce the shelf life of the food, however, so we were tasked with finding the right balance that maximizes both microbial control and freshness.”

Fresh vegetables and fruits respire inside the atmospherically limited packaging, consuming an already limited amount of oxygen and producing additional carbon dioxide. Eventually they deplete the package’s oxygen supply, at which point fermentation begins to take place, eventually rendering the produce both damaged and decidedly unappetizing.

Striking the right balance between controlling microbial growth with carbon dioxide and providing enough oxygen to extend the shelf life of the produce was the focus of Almenar’s work. Testing a wide range of gas combinations in celery, onions and tomatoes ultimately led to the discovery of the ideal mixtures that achieved both goals.

“Our work found that specific gas mixtures in conjunction with the right sanitizers can result in safer produce while maintaining the fresh quality that consumers desire,” Almenar said. “We have also seen that in-package gases are more effective than sanitizers at reducing microbial activity of produce.”

Consumer expectations key to acceptance

To ensure that the produce meets consumer expectations, Harte, associate professor in the Department of Food Science and Human Nutrition, conducted a series of surveys and sensory panels. Her team had consumers compare stored produce, such as celery and onions, with produce fresh from the field and rate them on texture, color, aroma and other characteristics.

“Even if the produce is safe, if it doesn’t appeal to consumers in the store, they won’t buy it,” Harte said. “It wasn’t enough that we simply controlled the microbial growth on the food — we had to make sure that the produce was marketable, that controls were something retailers could put into practice without hurting their sales.”

The research has resulted in a series of safer produce packaging protocols without sacrificing the freshness that makes fresh-cut produce an important part of a healthy diet.

“Assuming that industry adopts these new strategies, our produce will be safer and have a longer shelf life,” Ryser said. “There is, of course, no silver bullet when dealing with pathogens, but we can significantly inhibit their growth and spread, and keepthem from reaching dangerous levels.”